Colored finish ideas for climbing wall hand holds

I suggest you buy the hand holds from the same source as your climbing wall. They have probably thought through the matter carefully.

At least worth pricing mfd hand/ft holds. There are some products available that are intended to be mixed with floor paints/epoxy/etc to prevent slipping on wet floors. There are various types and could be mixed into poly or whatever finish you want. I think different grit sizes are available and you control how much you use to get the desired grip.

Straight dye will rub off, and it can be difficult to get bright intense color with dye. Recommend paint with a bit of abrasive. Also, just a "roughening up" of the surface, say a rasp or low grit sandpaper, could work. A coat of poly thinned 1:1, keep wet for 20 min, wipe off, will soak into and harden the wood so it doesnt wear as quickly, then paint bright colors over that. Paint wont harden the wood as much as poly.

I've made a few holds - out of wood. They work great, and are much cheaper than $20/apiece jugs. But for better grip on the slopey ones, I smeared some 2 part epoxy and sprinkled on clean crushed quartz (not playground sand). It leaves a good rough surface, but not one that will tear skin at a touch. I imagine you could just spray paint over that and still keep a good gripy surface.

I made some from glued up Baltic birch that are still good years later without finish. There used to be an online book on making climbing walls, but I think it may have been published and taken down in the interim.

I suspect brightly colored permanent markers would work to dye the wood without rubbing off or interfering with grip. That's what I would try.

Your three year old should have a lot of fun.